Class D amplifiers are increasingly being used in output stages of electronic devices for which power efficiency is important, such as mobile telephones, portable media players, laptop and tablet computers and wireless headphones, earphones and earbuds.
A Class D amplifier receives an analogue input signal and outputs a sequence of pulses, with the width and separation of pulses being representative of the amplitude of the input analogue signal.
Some Class D amplifiers may be designed to operate in Class AD mode, some Class D amplifiers may be designed to operate in Class BD mode.
In Class AD designs, the signal output at a point in time by the Class D amplifier can take one of two amplitude values, a high value and a low value. Thus, in Class AD mode the output signal is bi-level. This is shown schematically in the upper waveform diagram 110 of FIG. 1, which shows (in the lowermost waveform 112) a differential signal applied to a bridge-tied load by the output stage of a full bridge Class D amplifier, and corresponding output signals 114, 116 generated by first and second drivers of the output stage of the amplifier. In this example the output signals 114, 116 generated by the first and second drivers swing between a positive supply rail voltage +Vdd and a negative power supply rail voltage −Vdd, and these signals are always of opposite polarity, so the differential signal applied across the load can only be either +2Vdd or −2Vdd (assuming that +Vdd is equal in magnitude to −Vdd).
In Class BD mode, the signal output at a point in time by the Class D amplifier can take one of three amplitude values: a high value; a low value; and an intermediate value between the high value and the low value. Thus, in Class BD mode the output signal is tri-level. This is shown schematically in the lower waveform diagram 120 of FIG. 1, which shows (in the lowermost waveform 122) a differential signal applied to a bridge-tied load by the output stage of a full bridge Class D amplifier, and corresponding output signals 124, 126 generated by first and second drivers of the output stage of the amplifier.
In this example the output signals 124, 126 generated by the first and second drivers swing between a positive supply rail voltage +Vdd and a negative power supply rail voltage −Vdd. These outputs are sometimes of opposite polarity but also sometimes of the same polarity, and so the differential signal applied across the load can be either +2Vdd or −2Vdd (assuming that +Vdd is equal in magnitude to −Vdd) or 0V.
The choice of whether to design a particular Class D amplifier to operate in Class AD mode or Class BD mode depends upon the requirements of the particular application for which the Class D amplifier is to be used. Each mode offers different advantages and disadvantages.